U.S. Pat. No. 5,515,829 to Wear et al. describes a variable displacement actuating fluid pump for a hydraulically-actuated fuel injection system. In this system, a high pressure common rail supplies pressurized lubricating oil to a plurality of hydraulically-actuated fuel injectors mounted in a diesel engine. The common rail is pressurized by a variable displacement swash plate type pump that is driven directly by the engine. Pressure in the common rail is controlled in a two-fold manner. First, some pressure control is provided by electronically varying the swash plate angle within the pump. However, because variable angle swash plate type pumps typically have a relatively narrow band of displacement control, pressure in the common rail is primarily controlled through an electronically controlled pressure regulator. The pressure regulator returns a portion of the pressurized fluid in the common rail back to the low pressure fluid sump in order to maintain fluid pressure in the common rail at a desired magnitude.
While the Wear et al. hydraulically-actuated system using a variable displacement pump has performed magnificently for many years in a variety of diesel engines manufactured by Caterpillar, Inc. of Peoria, Ill., there remains room for improvement. On the overall level, the Wear et al. system is relatively more complex in that the control scheme in its electronic control module must simultaneously control both the angle of the swash plate within the high pressure pump and the amount of fluid spilled via the pressure regulator. Also, variable angle swash plate type pumps are relatively complex, and thus more prone to mechanical breakdown relative to simple fixed displacement type pumps. Finally, the Wear et al. system inherently wastes energy that inevitably results in a higher than necessary fuel consumption for the engine. In other words, energy is wasted each time the pressure regulator spills an amount of pressurized fluid back to the low pressure sump.
In another class of fuel injection systems, an individual unit pump supplies pressurized fuel to each individual fuel injector. In still another class of fuel injection systems, an individual unit pump supplies actuating fluid to each individual hydraulically-actuated fuel injector. In both of these classes of fuel injection systems, the individual unit pumps are of a fixed displacement and fixed output type such that a substantial amount of energy is wasted in those instances where an amount of fuel injected corresponds to less than the full stroke of the individual unit pumps. In most instances, these fuel injection systems utilize electronically controlled spill valves on the fuel injectors themselves in order to control injection timing and/or quantity. However, because of space constraints at and around the engine head, compromises must often be made in order to accommodate such a spill valve in or on the fuel injector body, as well as the associated plumbing, etc.
The present invention is directed to overcoming these and other problems associated with, and improving upon, fluid pumps and their associated hydraulically-actuated systems.